What is well done son in Welsh?

There isn't a direct equivalent to "well done son" in Welsh that carries the same emotional weight and familiarity. However, here are some options you could use depending on the context:

Formal:

* Da iawn, fab! - This literally translates to "Very good, son!" and is a suitable option for a more formal situation.

Informal:

* Craid! - This means "Great!" and is a common exclamation of praise.

* Da chi! - This literally translates to "Good you!" but is often used as a casual way of saying "Well done!"

* Ti'n rhyfeddol! - This means "You're amazing!" and is a more heartfelt and affectionate way to praise someone.

Adding "son":

You could add "mab" (son) to any of the above phrases, but it might sound a bit stiff. For example:

* Craid, mab! - This would sound like "Great, son!"

Important note: The best way to convey this phrase would be to use a combination of words and tone that reflects the specific relationship between you and the person you are addressing.